1. Field of the Invention
The present invention relates to novel granules of sodium metasilicate, a process for the preparation thereof, and the utilization of same in detergency applications, especially in detergent compositions designed for the cleaning of household dishes and like kitchenware.
2. Description of the Prior Art
Sodium metasilicate is a well-known product which is particularly suited for detergent compositions useful in dishwashers, providing the same with the required alkalinity while at the same time being less aggressive than the other alkaline salts. Indeed, in spite of its alkalinity, it is but slightly corrosive when in contact with glass and light metals such as aluminum. Furthermore, it displays the advantage of possessing interesting detergency properties, such as desirable emulsifying activity and is a potent anti-redeposition agent.
Hence, it is frequently incorporated into detergent compositions designed for cleaning dishes and the like. However, presently, solid detergent compositions for washing dishes in an automatic washing machine are manufactured according to the technique known as "dry mixture", to a point where the physiochemical characteristics of said compounds essentially correspond to those of the basic materials, since the method of mixing alters them either very slightly or not at all.
Raw materials, and especially sodium metasilicate, the most important one, must therefore provide an entire spectrum of often contradictory specifications. Thus, the sodium metasilicate must be provided in such fashion that it satisfies the following requirements:
(1) Since detergent compositions for dishwashers are distributed to a broad segment of the population, a good visual appearance is required which implies acceptable aesthetic whiteness, well-rounded grains having an appropriate average diameter, and a narrow granulometric distribution which excludes the finest particles and the largest ones as might generate a feeling of heterogeneity;
(2) Aside from the aforenoted visual and tactile characteristics, in order to be accepted by the user, detergent compositions for dishwashers must be dense, concentrated products. Therefore, the raw materials must have a high specific gravity, exceeding 1 and an optimized water content such as not to result in too great a dilution of the composition. The seemingly high density is also required due to the relatively small size of the mechanical distributors therefor which are incorporated into the typical dishwashing machines;
(3) The various raw materials must also be harmless and non-toxic to the user. However, sodium metasilicate might be considered a relatively cautic compound. One of its well-known disadvantages is that it gives rise to a temporary irritation of the user's mucous membranes which translates into tingling sensations in the nose, as well as sneezing. This respiratory affliction is caused by the generation of extremely fine sodium metasilicate particles that stem from erosion or destruction (attrition) of larger particles during the various mechanical handlings to which the product is subjected. Usually, one employs the expression "dusting" to define this phenomenon and the concept of dusting makes it possible to quantify the aggressiveness or corrosiveness of dishwashing formulae. One of the reasons sodium metasilicate is a relatively corrosive product stems from its alkalinity. And yet, such alkalinity becomes absolutely necessary as a result of the "effectiveness" imperative which is impossible to sacrifice, such that there is a problem that must be resolved in view of the dual requirements which are antagonistic or competing;
(4) The raw materials must be stable under storage and they must also be quite resistant to binding. Same must also be compatible with the other usual constituents of dishwashing detergent compositions. They must not effect any deterioration of the odorants, the chlorine-releasing ingredients (sodium dichloroisocyanurate), nor the non-ionic surface active agents.
Among the surfactants typically employed, representative are compounds prepared from ethylene oxide and polypropylene glycols or from ethylene oxide and the alkylphenols. Unfortunately, the compositions resulting from admixture of the typical detergent additives and anhydrous sodium metasilicate are not stable. One notes especially the emergence of coloring upon completion of a particular storage period, as well as the development of unacceptable odors, especially with an aldehyde base;
(5) Finally, the raw materials must be adopted for operative distribution in dishwashing machines. This latter point is one of the most significant and it embraces both the ability of the product to be evacuated from the automatic distributor, and to dissolve as quickly and as thoroughly as possible during the "washing" sequence of the machine cycle.
From the foregoing, it will be appreciated that it is extremely difficult to develop a raw material which enables the sodium metasilicate to meet all the previously mentioned imperatives that are often contradictory.
To date, the raw materials in question which are anhydrous sodium metasilicate, slightly hydrated sodium metasilicate, pentahydrated sodium metasilicate in the form of spheres, particulates or granulates, have not proven altogether satisfactory.
One can incorporate into detergent compositions for dishwashers an almost anhydrous sodium metasilicate which contains no more than 0.2% of water which results from the fusion of sodium carbonate and sand, followed by cooling, grinding and sifting, between 0.4 and 1.4 mm, for example.
The benefits from such a product are its extremely high apparent density which originates from its but slight porosity and its high concentration due to the extremely low water content, as well as the lack of binding risks as a result of greater temperatures and adequate compatibility with chlorine-releasing ingredients.
The disadvantages are far more prevalent: its appearance is that of a product which has been ground, the individual grains of which display sharp edges, and the whiteness of which is not clear and depends largely upon the raw materials because its method of manufacture does not include a purification step. Dusting is also a problem, and its compatibility is poor with non-ionic surface active agents. It is also a product which is chemically fairly caustic due to its anhydrous character and that, in the event of increased hygroscopicity, it can bind. Finally, its ability to dissolve in the dishwashing water too is poor.
In order to alleviate certain of the disadvantages of the anhydrous sodium metasilicate, it has been proposed to granulate same. One can prepare low water content granules from an aqueous sodium metasilicate solution having a molar ratio of SiO.sub.2 /Na.sub.2 O which equals 1 by granulating and then drying same. According to a technique which is described in French patent application No. 77/29001, published under No. 2,403,974, and U.S. Pat. No. 4,253,849, particulates having a water content of 2 to 6% of water are prepared via a method which entails contacting the anhydrous sodium metasilicate with a sodium metasilicate liquor, by atomizing said liquor onto the mass of anhydrous sodium metasilicate particles which are maintained in motion by the rotation of a drum which is equipped with lifting blades, thus generating the formation of particle clouds, the particles are then subjected to the thermal action of hot gases at a temperature ranging from 200.degree. to 500.degree. C. and, finally, the product granules are cooled within a particular zone, under agitation, having an average temperature ranging from 42 .degree. to 65.degree. C.
The technique immediately above-described provides an improved product. Its appearance and whiteness are better, and its concentration is decreased. Also, its compatibility is better with non-ionic surface active agents. Dusting and speed of dissolution too are significantly improved.
On the other hand, its apparent density is lower and grain hardness is weaker.
It has also been proposed to provide the sodium metasilicate as a pentahydrated sodium metasilicate, shaped as spheres or particulates. Usually, such a product is obtained from a sodium metasilicate solution having a molar ratio of SiO.sub.2 /Na.sub.2 O which is equal to 1, concentrating such solution to stoichiometric proportions, thence inducing crystallization by cooling on a cooled band and, ultimately, granulation via fluidized bed or turning drum.
The products are obtained by crystallization from solution, and thence by purification, e.g., through filtration. Hence, this is one of the advantages flowing from this type of product, where it can be made in a form which is more pure.
Pentahydrated sodium metasilicate displays a better behavior upon dissolution thereof. For example, it manifests a diluted and "stabilized" form of metasilicate; hence, it becomes less caustic than its anhydrous counterparts.
The resistance of such products to binding by increasing hygroscopicity improves, because same require less water. Also, such products display greater compatibility with non-ionic surfactants.
However, they also manifest many disadvantages. For example, the color of pentahydrated sodium metasilicate does not attain values on the same order as those of granular anhydrous sodium metasilicate, which is much whiter. Subsequently, in the detergent composition itself, heterogeneity of color is an inevitable result, and detracts from the appearance thereof.
Moreover, the apparent density thereof is much less than that of anhydrous sodium metasilicate, given equal grain porosity and average diameter. The amount of water in the pentahydrated sodium metasilicate is significant (42.5%); hence, use thereof results in too great a dilution of the dishwashing formulae and a corresponding reduction in effectiveness. In the event of increased temperature, the risks of binding by partial fusion are great.
Finally, compatibility is bad between pentahydrated sodium metasilicate and active chlorine-releasing additives.
From the foregoing, it is apparent that it is difficult to balance the requirements of the raw material providing the sodium metasilicate, on the one hand, which must satisfy certain economic objectives which mandate a concentrated, effective product, with, on the other hand, the requirements of harmlessness and non-corrosiveness with regard to the user.